In conventional automatic transmissions, air flows into and out of the transmission housing through a vent or a breather tube during transmission operation. The in and out flow of air can be characterized as transmission breathing. When temeratures in the transmission housing increase, the air and operating fluid in the transmission housing expand and air flows freely out of the vent into the atmosphere. Without a provision for breathing, pressure within the transmission housing would become sufficiently high to force transmission hydraulic operating fluid past the seals.
When the temperatures in the transmission housing decrease, as when a car with a hot transmission is driven through high water, air flows freely into the transmission housing through the vent. Without a provision for breathing, vacuum within the transmission housing would draw contaminants into the transmission through the transmission seals.
During transmission breathing, the air within the transmission housing readily mixes with the operating fluid and causes undesired operating fluid oxidation. The oxidation accelerates with increases in operating fluid temperatures and breathing rates. This requires the transmission operating fluid be changed periodically.
One method of minimizing the oxidation of the operating fluid is to restrict the breathing of the transmission. Such a breathing control system is described in U.S. Pat. No. 3,686,973 issued to Davison, Jr. et al. on Aug. 29, 1972 and assigned to the assignee of the present invention. The breathing control system is normally closed, preventing fresh air from entering the transmission under most operating conditions. The device does provide vacuum relief when a predetermined level of vacuum within the transmission housing is reached.
Restricting the breathing of the transmission can make accurate measuring of the transmission operating fluid level difficult. The use of a breather control system which restricts the exchange of air with atmosphere results in residual pressure or vacuum developing within the transmission housing. Most automatic transmissions use an ullage rod, or dipstick, for measuring operating fluid levels. The dipstick is disposed in an operating fluid fill pipe which serves to limit the travel of the dipstick into the transmission. The fill pipe has its lower end inserted into the transmission housing. The lower end of the fill pipe is submerged beneath the operating fluid level in the transmission.
The residual pressure and vacuum make the operating fluid level indicator on the dipstick an unreliable indicator of transmission operating fluid level. A positive gas pressure in the transmission housing would push the operating fluid in the partially submerged fill pipe to a level higher than that in the transmission housing. Thus, the operating fluid level indicated by the dipstick would not be the same as that in the transmission housing. For example, with a gas pressure of 0.02 psig in the transmission housing, the operating fluid level inside the fill pipe will be about 0.7 inches higher than that in the transmission housing. (Height of operating fluid in fill pipe=gas pressure in transmission housing divided by the operating fluid density). With a negative pressure (a vacuum) retained in the transmission housing, the dipstick reading would be similarly lower than that in the transmission housing. When the level of operating fluid indicated on the dipstick does not correspond to the actual level within the cavity, the level of operating fluid within the cavity cannot be determined with the dipstick.
Transmissions with both the bottom end of the fill pipe below the opearating fluid level in the transmission housing cavity and a breather control restricting the passage of gas between the interior of the transmission housing and atmosphere require relief of pressure and vacuum to the atmosphere in order to obtain an accurate dipstick reading of the operating fluid level in the transmission housing.